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  <span class="token keyword">return</span> <span class="token number">100</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Promise {&lt;resolved&gt;: 100}</span>
</code></pre></div><p>这就是隐式返回 Promise 的效果。</p> <h2 id="await"><a href="#await" class="header-anchor">#</a> await</h2> <p>我们来看看 await做了些什么事情。</p> <p>以一段代码为例:</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span>
  <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token number">200</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">200</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>
<span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">300</span><span class="token punctuation">)</span>
</code></pre></div><p>我们来分析一下这段程序。首先代码同步执行，打印出0，然后将test压入执行栈，打印出100, 下面注意了，遇到了关键角色await。</p> <p>放个慢镜头:</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">await</span> <span class="token number">100</span><span class="token punctuation">;</span>
</code></pre></div><p>被 JS 引擎转换成一个 Promise :</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> promise <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span>reject</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
   <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>这里调用了 resolve，resolve的任务进入微任务队列。</p> <p>然后，JS 引擎将暂停当前协程的运行，把线程的执行权交给父协程(父协程不懂是什么的，上上篇才讲，回去补课)。</p> <p>回到父协程中，父协程的第一件事情就是对await返回的Promise调用then, 来监听这个 Promise 的状态改变 。</p> <div class="language-js extra-class"><pre class="language-js"><code>promise<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">value</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token comment">// 相关逻辑，在resolve 执行之后来调用</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>然后往下执行，打印出300。</p> <p>根据<code>EventLoop</code>机制，当前主线程的宏任务完成，现在检查微任务队列, 发现还有一个Promise的 resolve，执行，现在父协程在then中传入的回调执行。我们来看看这个回调具体做的是什么。</p> <div class="language-js extra-class"><pre class="language-js"><code>promise<span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">value</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token comment">// 1. 将线程的执行权交给test协程</span>
  <span class="token comment">// 2. 把 value 值传递给 test 协程</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>Ok, 现在执行权到了test协程手上，test 接收到父协程传来的200, 赋值给 a ,然后依次执行后面的语句，打印200、200。</p> <p>最后的输出为:</p> <div class="language- extra-class"><pre class="language-text"><code>0
100
300
200
200
</code></pre></div><p>async函数对 Generator 函数的改进，体现在以下四点：</p> <ol><li>内置执行器。Generator 函数的执行必须依靠执行器，而 async 函数自带执行器，无需手动执行 next() 方法。</li> <li>更好的语义。async和await，比起星号和yield，语义更清楚了。async表示函数里有异步操作，await表示紧跟在后面的表达式需要等待结果。</li> <li>更广的适用性。co模块约定，yield命令后面只能是 Thunk 函数或 Promise 对象，而async函数的await命令后面，可以是 Promise 对象和原始类型的值（数值、字符串和布尔值，但这时会自动转成立即 resolved 的 Promise 对象）。</li> <li>返回值是 Promise。async 函数返回值是 Promise 对象，比 Generator 函数返回的 Iterator 对象方便，可以直接使用 then() 方法进行调用。</li></ol> <p>总结一下，<code>async/await</code>利用<code>协程</code>和<code>Promise</code>实现了同步方式编写异步代码的效果，其中Generator是对协程的一种实现，虽然语法简单，但引擎在背后做了大量的工作，我们也对这些工作做了一一的拆解。用async/await写出的代码也更加优雅、美观，相比于之前的Promise不断调用then的方式，语义化更加明显，相比于co + Generator性能更高，上手成本也更低，不愧是JS异步终极解决方案</p></div> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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